1. Field of the Invention
This invention relates to a process for producing a tetraalkoxysilane. More particularly it relates to an improved process for producing a tetraalkoxysilane by subjecting silicon and a lower alkyl alcohol to a catalytic reaction in the presence of an alkali metal alkoxide catalyst.
2. Description of the Related Art
Heretofore, as a commercial production process of tetraalkoxysilanes, there is a process utilizing the dehydrochlorination reaction of SiCl.sub.4 with alcohols. According to the process, however, polymer formation as a side reaction occurs due to a by-product, hydrogen chloride, and also there exists, for example, a problem that steps must be taken for preventing corrosion of reaction equipment due to the presence of hydrogen chloride.
Thus, as a production process of tetraalkoxysilanes having overcome such a problem to a certain extent, there is also known a process of directly reacting silicon with alcohols in the presence of an alkali metal alkoxide. For example, there is known a process of A.Lenz et al using sodium methylate (Japanese patent publication No. 45-8217/1970), a process of W. Flick et al using a compound selected from ether alcohols and alkanolamines (Japanese patent application laid-open No. 54-138523/1979) and a process of Delaval et al using a waste matter powder formed in the direct preparation of methyl (or phenyl) chlorosilane containing at least 30 % of silicon and also using as a diluent, an aromatic hydrocarbon having a boiling point of 190.degree. C. or higher and a melting point of 40.degree. C. or lower (French patent No. 2,332,993).
Now, when a tetraalkoxysilane is produced by reacting silicon with alcohols in the presence of an alkali metal alkoxide, the following conditions are desired:
(1) a condition that the alkali metal alkoxide catalyst is very soluble in the reaction mixture; PA0 (2) a condition that a desired reaction temperature can be maintained; PA0 (3) a condition that a wide range of silicon-containing materials from fine particles to coarse particles can be used as raw material; PA0 (4) a condition that when coarse particles of silicon are used, the reaction rate is not significantly slower as compared with use of fine particles of silicon; PA0 (5) a condition that the yield of the tetraalkoxysilane based on silicon as a raw material is high; PA0 (6) a condition that continuation and termination of the reaction can be controlled; and PA0 (7) a condition that the tetraalkoxysilane can be produced on a commercial scale and preferably continuously. PA0 (1) By adding a specified ether compound, the alkali metal alkoxide dissolves well in the reaction mixture; deposition of the alkali metal alkoxide and its adhesion to the wall of the reaction vessel during the reaction are prevented; and it is possible to raise the reaction rate. PA0 (2) By selecting a high boiling ether compound, it is possible to carry out the reaction at high temperatures. For example, in the case where methanol having a boiling point of 65.degree. C. is used as a raw alcohol, it is possible to maintain the reaction temperature at about 120.degree. C. by selecting triethylene glycol dimethyl ether as an ether compound. PA0 (3) By distilling off continuously the formed tetraalkoxysilane in the reaction mixture together with unreacted lower alkyl alcohol therein during the reaction, it is possible to easily recover these compounds. PA0 (4) As to the silicon used in the present invention, those having a relatively large particle diameter can be used; hence it is possible to simplify the operations of grinding and classifying the silicon. PA0 (5) The yield of the tetraalkoxysilane relative to the silicon used is high, and the quantity of by-products other than hydrogen is neglegibly small. PA0 (6) In the case where the reaction is carried out by continuously feeding a lower alkyl alcohol to the reaction system, it is possible to discontinue or restart the reaction by discontinuing or restarting the feed of the lower alkyl alcohol. PA0 (7) By continuously feeding silicon and a lower alkyl alcohol to the reaction system and distilling off the resulting tetraalkoxysilane, etc. from the reaction system, it is possible to continuously commercially produce the tetraalkoxysilane.
According to an embodiment of the above process of A. Lenz et al, a 38 % solution (1 Kg) of sodium methylate is added to ferrosilicon in the form of fine particles (silicon, 90 % by weight; particle diameter, about 10 .mu.m), followed by reacting the mixture at 100.degree. C. for 2 hours, so that sodium methylate crystallizes at the final period of the reaction; thus in order to dissolve it, fresh methanol (195 g) is further added. However, according such a process, the solubility of the alkali metal alkoxide in the reaction mixture is low. On the other hand, another embodiment of the process of A. Lenz et al discloses that when the reaction is conducted using a 15 % solution of sodium methylate, addition of fresh methanol at the final period of the reaction after 4 hours is unnecessary, but in order to raise the solubility of the alkali metal alkoxide, a long time is required.
Further, according to still another embodiment of the process of A. Lenz et al, when ferrosilicon in the form of coarse particles (silicon, about 90 % by weight; average particle diameter, about 1 cm) is used, a reaction time as long as 280 hours is required and the use of the sodium methylate catalyst is very large.
On the other hand, according to an embodiment of the above process of W. Flick et al, when purified silicon (Si, 99.8.about.100% by weight; particle diameter &lt;10.mu.), tetraethoxysilane, ethanol, sodium ethoxide and ethylglycol are reacted at a reaction temperature of 140.degree. C. to 155.degree. C., the quantity of hydrogen generated is reduced to about 1/20 of that at the initial period of the reaction, so that the reaction temperature must be maintained by gradually reducing the ethanol feed during the progress of the reaction; hence operation is troublesome.
According to an embodiment of the above process of Delaval et al, silicon having a purity of about 72 %, methanol, tetramethoxysilane, sodium methoxide and an aromatic hydrocarbon such as diisopropylbenzene as a diluent are reacted at 120.degree. C. However, according to an experiment of the present inventors, when silicon having a purity of 94 %, methanol, tetramethoxysilane, sodium methoxide and tetralin or dibenzyltoluene as a diluent were reacted, a portion of sodium methoxide adheres to the wall of the reaction vessel together with silicon, and the reaction rate of silicon having an average particle diameter of 100 .mu.m was low (see Comparative examples 1 and 2 mentioned later).